Nonalcoholic fatty liver disease (NAFLD) and type-2 diabetes (T2D) are associated with increased risk of hepatocellular carcinoma (HCC), and individuals with T2D treated with metformin have a reduced risk of HCC incidence and mortality. Although several factors have been reported to probably mediate the preventive effect of metformin on the risk of several types of cancers, a precise mechanism by which metformin reduces HCC risk remains largely unknown. Specifically, it is not known what cellular factors, targeted by metformin, are critical in the development of HCC in diabetic patients, and which sub- population of patients may be more responsive to metformin treatment. Our long-term goal is to understand the molecular basis of HCC. The objective of this proposal is to unveil the mechanism by which metformin inhibits spontaneous HCC development in mice bearing a heterozygous deletion of the nuclear receptor coactivator-5 (Ncoa5) gene and improve current tools utilizing identified genes and signatures to predict prognosis and response to metformin in patients with HCC. The central hypothesis is that reduced Ncoa5 expression is a key risk factor for HCC development in the context of a diabetic condition, and metformin protects mice against HCC development by reducing hepatic insulin resistance and steatosis, in part by positively regulating the expression of Ncoa5. The rationale for the proposed research is that therapeutic strategies, aimed at increasing Ncoa5 expression or targeting Ncoa5 deficiency driven oncogenic pathways, are potentially effective in protecting against HCC. This hypothesis will be tested by two specific aims: 1) determine the effect of metformin on HCC development and Ncoa5 expression in the liver using a novel genetically-engineered mouse model (Ncoa5+/- mice) of HCC, and 2) identify and translate critical targets and pathways, in metformin-mediated protection against HCC, from a mouse model of HCC to human HCC. The approach is innovative because it utilizes a novel genetically-engineered Ncoa5+/- mouse model of HCC to analyze the effects of metformin on hepatocarcinogenesis and identifies new genes and gene signatures for predicting the prognosis of patients with HCC. The proposed research is significant because it is expected to establish a new concept that metformin inhibits an Ncoa5 deficiency-induced pathogenic pathway that is commonly shared by NAFLD, T2D and HCC. This will not only advance our understanding of the etiological mechanisms of HCC, but also provide better biomarkers for predicting the risk of recurrence and response to metformin related therapy in patients with HCC.